Aqueous coating materials and methods of making and using same

ABSTRACT

The invention relates to an aqueous coating material. The aqueous coating material comprises 5-95% by weight (based on the overall binder content) of a binder component a) and 5-95% by weight (based on the overall binder content) of a binder component b). The binder component a) is obtainable by free-radical polymerization of an aqueous dispersion of a polyurethane resin with ethylenically unsaturated monomers and in the presence of a water-insoluble initiator. The binder component b) is obtainable by polymerizing ethylenically unsaturated monomers in an organic solvent or in a mixture of organic solvents and in the presence of a polyurethane resin and converting the resulting binder component b) into an aqueous dispersion.

This application is the national stage of International Application No.PCT/EP98/02952, filed May. 20, 1998.

FIELD OF THE INVENTION

The invention relates to an aqueous coating material and to the use ofsuch a material in a process for producing two-coat paint systems.

BACKGROUND ART

Aqueous coating materials are used in particular to produce two-coatautomotive topcoat systems of the basecoat-clearcoat type. In suchsystems, coating materials according to the invention are particularlythe basecoat materials or basecoats. The quality of a paint systemobtainable by a process for producing two-coat paint systems dependscritically on the aqueous basecoat employed.

EP-A-353 797 describes aqueous coating materials which can be used asbasecoats. The binder present in these known coating materials is apolymer obtainable by subjecting acrylate and/or methacrylate monomersto an emulsion polymerization, initiated by means of water-solubleinitiators in the presence of an anionic polyurethane resin, which mayalso contain vinyl groups. If the aqueous coating materials described inEP-A-353 797 are used as basecoats in the process referred to above, theresulting two-coat paint systems are unsatisfactory in their stabilityto condensed moisture. This disadvantage is evident in particular withrefinishes, which are cured only at temperatures of up to 80° C.Furthermore, the aqueous coating materials described in EP-A-353 797display unsatisfactory stability on storage if they include a melamineresin as additional binder component.

EP-A-297 576 describes a process for producing two-coat paint systems ofthe type described above in which the basecoat used comprises aqueouscoating materials comprising an aqueous polymer dispersion obtainable bypolymerizing ethylenically unsaturated monomers in an aqueous dispersionin the presence of a polyurethane resin which contains urea groups butno vinyl groups. If the aqueous coating materials described in EP-A-297576 are used as basecoats in the process referred to above for producingtwo-coat paint systems, then the two-coat paint systems obtained are inneed of improvement in terms of their resistance to condensed water.Furthermore, the aqueous coating materials described in EP-A-297 576 arefrequently observed to show deficient stability on storage and defectswhose root cause is incompatibility, if combinations of differentbinders are used.

DE-A-40 10 176 describes a process for producing a two-coat paint systemusing an aqueous basecoat comprising as binder a polymer obtainable bypolymerizing ethylenically unsaturated monomers in an organic solvent inthe presence of a polyurethane resin containing polymerizable doublebonds and converting the resultant reaction product into an aqueousdispersion. If basecoats containing metal pigments are used in theprocess described in DE-A-40 10 176, the resulting two-coat metallicsystems are in need of improvement in terms of their metal effect.Finally, the stone-chip resistance is also capable of improvement.

The reference DE P 4339870.7 discloses an aqueous coating material basedon an aqueous dispersion of a polyurethane resin which is polymerizedwith ethylenically unsaturated monomers in the presence of awater-insoluble initiator. A lack of sufficient shear stability meansthat this known coating material is poorly suited to the dispersion ofpigments.

The references EP 522419 and EP 522420 disclose the polymerization ofmonomers in the presence of polyurethane dispersions containingpolymerizable double bonds. In accordance with these references, thepolymerization is conducted using water-soluble initiators, resulting inunsatisfactory stability on storage of coating materials prepared usingsuch binders. The polymerization, moreover, is carried out in theaqueous phase, leading to binders of comparatively poor suitability forthe dispersion of pigments.

The technical problem on which the invention is based is to provide anovel aqueous coating material for producing two-coat paint systems ofthe type referred to above, with which paint systems are obtained whosetechnical properties are improved in comparison with the prior art andwhich, in particular, feature the above-described disadvantages of theprior art to a reduced extent, if at all.

SUMMARY OF THE INVENTION

This object is surprisingly achieved by means of an aqueous coatingmaterial which comprises 5-95% by weight (based on the overall bindercontent) of a binder component a) and 5-95% by weight (based on theoverall binder content) of a binder component b), the binder componenta) being obtainable by subjecting an ethylenically unsaturated monomeror a mixture of ethylenically unsaturated monomers to free-radicalpolymerization in an aqueous dispersion of a polyurethane resin whichhas a number-average molecular weight of from 1000 to 30,000 andcontains on average from 0.05 to 1.1 polymerizable double bonds permolecule and in the presence of a water-insoluble initiator or of amixture of water-insoluble initiators, the weight ratio between thepolyurethane resin and the ethylenically unsaturated monomer or mixtureof ethylenically unsaturated monomers being between 1:10 and 10:1, andthe binder component b) being obtainable by subjecting (A) anethylenically unsaturated monomer or a mixture of ethylenicallyunsaturated monomers to polymerization in an organic solvent or in amixture of organic solvents and in the presence of (B) a polyurethaneresin having a number average molecular weight of from 200 to 30,000 andcontaining on average from 0.05 to 1.1 polymerizable double bonds permolecule and converting the resulting binder component b) into anaqueous dispersion.

In addition to binder components a) and b), further binder componentsmay also be present in the coating material. In particular, crosslinkingagents such as, for example, melamine resins may be presentadditionally. The aqueous coating material is prepared by mixing the two(aqueous) binder components a) and b), with or without further customaryadditives or auxiliaries, and a crosslinking agent and then combiningthem homogeneously by stirring or the like.

The basic concept of the invention consists, accordingly, in thecombination of two binders known per se from the prior art.Surprisingly, such a combination of these binders, used and developedper se as individual components, in one coating material displays aconsiderable synergistic effect in terms of the technical properties ofa coating system produced with a coating material of the invention. Ithas in fact been found that such a coating system of the inventionexhibits an excellent stone-chip resistance, in particular, incomparison with the prior art. The advantageous technical properties ofsuch systems relate both to production-line finishes (OEM finishes) andto refinishes.

DETAILED DESCRIPTION

In one preferred embodiment of the invention the polyurethane resin ofthe binder component a) comprises acrylate, methacrylate and/or allylether groups as groups containing polymerizable double bonds. It isadvantageous for the polyurethane resin of the binder component a) to beanionic and to have an acid number of from 20 to 60. As ethylenicallyunsaturated monomers of the binder component a) it is preferred to use amixture of (i) from 40 to 100% by weight of an aliphatic orcycloaliphatic ester of acrylic acid or methacrylic acid containingneither hydroxyl nor carboxyl groups, or a mixture of such esters, (ii)from 0 to 30% by weight of an ethylenically unsaturated monomer whichcarries at least one hydroxyl group in the molecule, or a mixture ofsuch monomers, (iii) from 0 to 10% by weight of an ethylenicallyunsaturated monomer which carries at least one carboxyl group in themolecule, or a mixture of such monomers, (iv) from 0 to 50% by weight ofan ethylenically unsaturated monomer other than (i), (ii) and (iii), ora mixture of such monomers, and (v) from 0 to 5% by weight of anethylenically polyunsaturated monomer or a mixture of such monomers, thesum of the weight fractions of (i), (ii), (iii), (iv) and (v) alwaysbeing 100% by weight.

In terms of the binder component b), the following developments arejudicious and/or advantageous. The polymer of the binder component b),which is prepared from (A) and (B), can have an acid number of from 5 to200, preferably from 10 to 40, with particular preference from 15 to 30,a hydroxyl number of from 0 to 100, preferably from 20 to 80, and anumber-average molecular weight of from 2000 to 20,000, preferably from5000 to 12,000. The polymer of the binder component b) prepared from (A)and (B) advantageously has a polydispersity index Q=M_(w) :M_(n) of from5 to 90, preferably from 10 to 30. The polymer of the binder componentb) is obtainable using components (A) and (B) in a weight ratio of from1:10 to 10:1, preferably from 1:2 to 2:1, with particular preference1:1. As component (B) of the binder component b) it is possible to use apolyurethane resin containing polymerizable double bonds which have beenintroduced by incorporating compounds containing not only apolymerizable double bond but also at least two NCO-reactive groups intothe molecules of the polyurethane resin. Specifically, the polyurethaneresin which can be used as component (B) of the binder component b)comprises allyl ether groups, especially trimethylolpropane monoallylether, as polymerizable double bonds. It is also preferred for thepolymer of the binder component b) to be obtainable using as component(B) a polyurethane resin which has an acid number of from 0 to 2.0.Specifically, it is desirable for the polymer of the binder component b)to be obtainable using as component (B) a polyurethane resin which has anumber-average molecular weight of from 1000 to 5000 and contains onaverage from 0.2 to 0.9, preferably from 0.3 to 0.7, polymerizabledouble bonds per molecule.

An aqueous coating material of the invention generally comprisespigments, especially solid-color pigments. Of course, however, effectpigments such as metal pigments and/or mica pigments may also beincorporated in addition. In principle, however, a coating material ofthe invention can also be formulated as a clearcoat and hence beintended for application to a pigmented basecoat.

The invention additionally relates to the use of an aqueous coatingmaterial of the invention as the pigmented basecoat in a process forproducing a twocoat paint system on a substrate surface, in which (1)the basecoat is applied to the substrate surface, (2) a polymer film isformed from the basecoat applied in stage (1), (3) a transparent topcoatis applied to the resulting basecoat film, and subsequently (4) thebasecoat film is baked together with the topcoat film, and also to theuse of a coating material of the invention for coating motor vehiclebodies or motor vehicle bodywork parts.

The preparation of the binder component a) is described in detail below.The aqueous dispersion of the polyurethane resin required for thispurpose can be prepared by using (a1) a polyesterpolyol and/orpolyetherpolyol having a number-average molecular weight of from 400 to5000 or a mixture of such polyesterpolyols and/or polyetherpolyols, and(a2) a polyisocyanate or a mixture of polyisocyanates, together ifdesired with a monoisocyanate or a mixture of monoisocyanates, and (a3)a compound which has in the molecule at least one isocyanate-reactivegroup and at least one group which is capable of forming anions, or amixture of such compounds, or (a4) a compound which contains in themolecule at least one NCO-reactive group and at least onepoly(oxyalkylene) group, or a mixture of such compounds, or (a5) amixture of components (a3) and (a4), and (a6) if desired, a compoundwhich contains not only a polymerizable double bond but also at leastone NCO-reactive group, or a mixture of such compounds, and (a7), ifdesired, a hydroxyl- and/or amino-containing organic compound having amolecular weight of from 60 to 399 or a mixture of such compounds toprepare a polyurethane resin which has a number-average molecular weightof from 1000 to 30,000, preferably from 1500 to 20,000, and whichcontains on average from 0.05 to 1.1, preferably from 0.2 to 0.9,polymerizable double bonds, and dispersing this resin in water.

The polyurethane resin for the binder component a) can be preparedeither in bulk or in organic solvents.

The polyurethane resin for the binder component a) can be prepared byreacting all starting compounds simultaneously. In many cases, however,it is judicious to prepare the polyurethane resin in stages. Thus it ispossible, for example, to use components (a1) and (a2) to prepare aprepolymer which contains isocyanate groups and is then reacted furtherwith component (a3) or (a4) or (a5). It is also possible to usecomponents (a1) and (a2) and (a3) or (a4) or (a5) and, if desired, (a6)to prepare a prepolymer which contains isocyanate groups and can then bereacted with component (a7) to give a polyurethane resin of highermolecular mass. The reaction with component (a7) can be carried out inbulk or--as described, for example, in EP-A-297 576--in water. In caseswhere a compound used as component (a6) contains only oneisocyanate-reactive group, it is possible in a first stage to use (a2)and (a6) to prepare a precursor which contains isocyanate groups and canthen be reacted further with the other components.

The reaction of the components (a1) to (a7) can also be carried out inthe presence of catalysts, such as dibutyltin dilaurate, dibutyltinmaleate and tertiary amines, for example.

The amounts of component (a1), (a2), (a3), (a4), (aS), (a6) and (a7) tobe used are a function of the target number-average molecular weight andof the target acid number. The polymerizable double bonds can beintroduced into the polyurethane molecules by using components (a1)having polymerizable double bonds and/or components (a2) havingpolymerizable double bonds and/or by means of the component (a6). It ispreferred to introduce the polymerizable double bonds by way of thecomponent (a6). It is also preferred to introduce into the polyurethaneresin molecule acrylate, methacrylate or allyl ether groups as groupscontaining polymerizable double bonds.

As component (a1) it is possible to use saturated and unsaturatedpolyester- and/or polyetherpolyols, especially polyester- and/orpolyetherdiols having a number-average molecular weight of from 400 to5000. Examples of suitable polyetherdiols are polyetherdiols of thegeneral formula H(--O--(CHR¹)_(n) --)_(m) OH in which R¹ is hydrogen ora lower, substituted or unsubstituted alkyl radical, n is from 2 to 6,preferably from 3 to 4, and m is from 2 to 100, preferably from 5 to 50.Examples are linear or branched polyetherdiols such as poly(oxyethylene)glycols, poly(oxypropylene) glycols and poly(oxybutylene) glycols. Thepolyetherdiols selected should not introduce excessive quantities ofether groups, since otherwise the polymers formed swell in water. Thepreferred polyetherdiols are poly(oxypropylene) glycols in the molarmass range Mn from 400 to 3000.

Polyesterdiols are prepared by esterifying organic dicarboxylic acids ortheir anhydrides with organic diols or are derived from ahydroxycarboxylic acid or a lactone. In order to prepare branchedpolyesterpolyols it is possible to use a minority of polyols orpolycarboxylic acids having a higher functionality. The dicarboxylicacids and diols can be linear or branched aliphatic, cycloaliphatic oraromatic dicarboxylic acids or diols.

The diols used to prepare the polyesters consist, for example, ofalkylene glycols, such as ethylene glycol, propylene glycol, butyleneglycol, 1,4-butanediol, 1,6-hexanediol, neopentylglycol and other diols,such as dimethylolcyclohexane. It is also possible, however, to addsmall amounts of polyols, such as trimeth-ylolpropane, glycerol andpentaerythritol. The acid component of the polyester consists primarilyof low molecular mass dicarboxylic acids or their anhydrides having 2 to44, preferably 4 to 36, carbon atoms in the molecule. Examples ofsuitable acids are o-phthalic acid, isophthalic acid, terephthalic acid,tetrahydrophthalic acid, cyclohexanedicarboxylic acid, succinic acid,adipic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid,glutaric acid, hexachloro-heptanedicarboxylic acid, tetrachlorophthalicacid and/or dimerized fatty acids. Instead of these acids it is alsopossible to use their anhydrides, where they exist. In the formation ofpolyesterpolyols it is also possible for relatively small amounts ofcarboxylic acids having 3 or more carboxyl groups to be present,examples being trimellitic anhydride or the adduct of maleic anhydridewith unsaturated fatty acids.

It is also possible to use polyesterdiols obtained by reacting a lactonewith a diol. They are notable for the presence of terminal hydroxylgroups and repeating polyester units of the formula (-CO--(CHR²)_(n)--CH₂ --O). In this formula, n is preferably from 4 to 6 and thesubstituent R² =hydrogen or an alkyl, cycloalkyl or alkoxy radical. Nosubstituent contains more than 12 carbon atoms. The total number ofcarbon atoms in the substituent does not exceed 12 per lactone ring.Examples thereof are hydroxycaproic acid, hydroxy-butyric acid,hydroxydecanoic acid and/or hydroxy-stearic acid.

For the preparation of the polyesterdiols preference is given to theunsubstituted epsilon-caprolactone, where n has the value 4 and allsubstituents R2 are hydrogen. The reaction with lactone is initiated bymeans of low molecular mass polyols such as ethylene glycol,1,3-propanediol, 1,4-butanediol and dimethylolcyclohexane. However, itis also possible to react other reaction components, such asethylenediamine, alkyldialk-anolamines or else urea, with caprolactone.Further suitable diols of relatively high molecular mass arepolylactamdiols, which are prepared by reacting, for example,epsilon-caprolactam with low molecular mass diols.

If polymerizable double bonds are to be introduced into the polyurethanemolecules by way of component (a1), then it is necessary to usecomponents (a1) which contain polymerizable double bonds. Examples ofsuch components (a1) are polyesterpolyols, preferably polyesterdiols,which have been prepared using polyols or polycarboxylic acidscontaining polymerizable double bonds, preferably polyols containingpolymerizable double bonds. Examples of polyols containing polymerizabledouble bonds are trimethylolpropane monoallyl ether, glycerol monoallylether, pentaerythritol monoallyl ether and pentaerythritol diallylether.

As component (a2) it is possible to use aliphatic and/or cycloaliphaticand/or aromatic polyisocyanates. Examples of aromatic polyisocyanatesare phenylene diisocyanate, tolylene diisocyanate, xylylenediisocyanate, biphenylene diisocyanate, naphthylene diisocyanate, anddiphenylmethane diisocyanate.

Because of their good resistance to ultraviolet light, (cyclo)aliphaticpolyisocyanates give rise to products having little tendency to yellow.Examples of cycloaliphatic polyisocyanates are isophorone diisocyanate,cyclopentylene diisocyanate, and the hydrogenation products of thearomatic diisocyanates, such as cyclohexylene diisocyanate,methylcyclohexylene diisocyanate and dicyclohexylmethane diisocyanate.Aliphatic diisocyanates are compounds of the formula OCN--(CR³ ₂)_(r)--NCO, in which r is an integer from 2 to 20, in particular from 6 to 8,and R³, which can be identical or different, is hydrogen or a loweralkyl radical of 1 to 8 carbon atoms, preferably 1 or 2 carbon atoms.Examples thereof are trimethylene diisocyanate, tetramethylenediisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate,propylene diisocyanate, ethylethylene diisocyanate, dimethylethylenediisocyanate, methyltrimethylene diisocyanate and trimethylhexanediisocyanate. A further example of an aliphatic diisocyanate istetramethylxylene diisocyanate.

Particularly preferred diisocyanates used are hexamethylenediisocyanate, isophorone diisocyanate, tetramethylxylene diisocyanateand dicyclohexylmethane diisocyanate.

In terms of the functionality of the polyisocyanates, the composition ofcomponent (a2) must be such that no crosslinked polyurethane resin isobtained. In addition to diisocyanates, component (a2) may also includea fraction of polyisocyanates having functionalities of more thantwo--triisocyanates, for example.

Products which have been found suitable for use as triisocyanates arethose resulting from the trlmerization or oligomerization ofdiisocyanates or from reaction of diisocyanates with polyols orpolyarnines. Examples of these include the biuret of hexamethylenediisocyanate and water, the isocyanurate of hexamethylene diisocyanate,or the adduct of isophorone diisocyanate with trimethylolpropane. Theaverage functionality can be lowered, if desired, by addingmonoisocyanates. Examples of such chain-terminating monoisocyanates arephenyl isocyanate, cyclohexyl isocyanate,1-(1-isocyanato-1-methylethyl)3-(1-methylethenyl)benzene, and stearylisocyanate.

In order to be able to convert the polyurethane resin in question to astable dispersion in water, it must contain hydrophilic groups. Thesehydrophilic groups are introduced into the polyurethane resin by meansof the component (a3) or the component (a4) or the component (a5). Thegroups of component (a3) which are capable of forming anions areneutralized with a base, preferably a tertiary amine, such asdimethylethanolamine, triethylamine, tripropylamine and tributylamine,or else aminomethylpropanol, for example, prior to or during thedispersion of the polyurethane resin in water, so that afterneutralization the polyurethane resin contains anionic groups. Where thecomponent (a3) is used exclusively as the component supplyinghydrophilic groups, component (a3) is used in an amount such that thepolyurethane resin has an acid number of from 15 to 80, preferably from20 to 60. Where the component (a4) is used exclusively as the componentsupplying hydrophilic groups, component (a4) is used in an amount suchthat the polyurethane resin contains from 5 to 40, preferably from 10 to30, % by weight of oxyalkylene groups, including if appropriateoxyalkylene groups introduced by means of the component (a1). Where thecomponent (a5) is used as the component supplying hydrophilic groups,the amounts of component (a3) and (a4) to be used are situated, inaccordance with the mixing ratio, between the values indicated above forthe cases in which component (a3) or (a4), respectively, is used as thesole supplier of hydrophilic groups. In any case, the skilled worker caneasily determine the amounts of components (a3), (a4) or (a5) to be usedby means of simple routine experiments. He or she need only test, bymeans of simple serial experiments, how high the proportion ofhydrophilic groups need be, at least, in order to obtain a stableaqueous polyurethane resin dispersion. He or she can of course also usecommon dispersing auxiliaries, such as emulsifiers, for example, inorder to stabilize the polyurethane resin dispersions. The use ofdispersing auxiliaries, however, is not preferred, since it generallyincreases the sensitivity of the resulting coating systems to moisture.

As component (a3) it is preferred to use compounds which contain twoisocyanate-reactive groups in the molecule. Suitable isocyanate-reactivegroups are, in particular, hydroxyl groups, and also primary and/orsecondary amino groups. Suitable groups capable of forming anions arecarboxyl groups, sulfonic acid groups and/or phosphonic acid groups,carboxyl groups being preferred. As component (a3) it is possible, forexample, to use alkanoic acids having two substituents on the α carbonatom. The substituent can be a hydroxyl group, an alkyl group or,preferably, an alkylol group. These alkanoic acids have at least one,generally from 1 to 3, carboxyl groups in the molecule. They have from 2to about 25, preferably from 3 to 10, carbon atoms. Examples ofcomponent (a3) are dihydroxypropionic acid, dihydroxysuccinic acid anddihydroxybenzoic acid. A particularly preferred group of alkanoic acidsare the α,α-dimethylolalkanoic acids of the general formula R⁴ --C(CH₂OH)₂ COOH, where R⁴ is a hydrogen atom or an alkyl group having up toabout 20 carbon atoms.

Examples of such compounds are 2,2-dimethylolacetic acid,2,2-dimethylolpropionic acid, 2,2-dimethylolbutyric acid and2,2-dimethylolpentanoic acid. The preferred dihydroxyalkanoic acid is2,2-dimethylolpropionic acid. Examples of compounds containing aminogroups are α,ω-diaminovaleric acid, 3,4-diaminobenzoic acid,2,4-diaminotoluenesulfonic acid and 2,4-diaminodiphenyl ether sulfonicacid.

With the aid of component (a4) it is possible to introducepoly(oxyalkylene) groups as nonionic stabilizing groups into thepolyurethane molecules. As component (a4) it is possible, for example,to use the following: alkoxypoly(oxyalkylene) alcohols having thegeneral formula R'O--(--CH₂ --CHR"O)_(n) H, in which R' is an alkylradical having 1 to 6 carbon atoms, R" is a hydrogen atom or an alkylradical having 1 to 6 carbon atoms, and n is a number between 20 and 75.

Component (a6) is used to introduce polymerizable double bonds into thepolyurethane resin molecules. It is preferred to use as component (a6) acompound which contains at least one NCO-reactive group and onepolymerizable double bond. Particular preference is given to using, ascomponent (a6), compounds which in addition to a polymerizable doublebond also contain two NCO-reactive groups. Examples of NCO-reactivegroups are --OH, --SH, >NH and --NH₂ groups, preference being given to--OH, >NH and NH₂ groups. Examples of compounds which can be used ascomponent (a6) are hydroxy (meth)acrylates, especially hydroxyalkyl(meth)acrylates such as hydroxyethyl, hydroxypropyl, hydroxybutyl orhydroxyhexyl (meth)acrylate and 2,3-dihydroxypropyl (meth)acrylate,2,3-dihydroxypropyl monoallyl ether, 2,3-dihydroxypropanoic acid allylester, glycerol mono(meth)acrylate, glycerol monoallyl ether,pentaerythritol mono(meth)acrylate, pentaerythritol di(meth)acrylate,pentaerythritol monoallyl ether, oentaerythritol diallyl ether,trimethylolpropane monoallyl ether, trimethylolpropanemono(meth)acrylate and trimethylolpropane diallyl ether. As component(a6) it is preferred to use trimethylolpropane monoallyl ether, glycerolmono(meth)acrylate, pentaerythritol di(meth)acrylate, pentaerythritoldiallyl ether, glycerol monoallyl ether and trimethylolpropanemono(meth)acrylate. As component (a6) it is particularly preferred touse trimethylolpropane monoallyl ether, glycerol monoallyl ether and2,3-dihydroxypropanoic acid allyl ester. It is preferred to incorporatethose components (a6) which contain at least two NCO-reactive groupsinto the polyurethane molecules in chain positions (not terminalpositions).

As component (a7) it is possible, for example, to use polyols having upto 36 carbon atoms per molecule such as ethylene glycol, diethyleneglycol, triethylene glycol, 1,2-propanediol, 1,3-propanediol,1,4-butanediol, 1,2-butylene glycol, 1,6-hexanediol, trimethylolpropane,castor oil or hydrogenated castor oil, ditrimethylolpropane ether,pentaerythritol, 1,2-cyclohexanediol, 1,4-cyclohexanedimethanol,bisphenol A, bisphenol F, neopentyl glycol, neopentyl glycolhydroxypivalate, hydroxyethylated or hydroxypropylated bisphenol A,hydrogenated bisphenol A and mixtures thereof. The polyols are generallyused in amounts of up to 30 percent by weight, preferably from 2 to 20percent by weight, based on the amount of component (a1) and (a7)employed.

As component (a7) it is also possible to use diamines and/or polyamineshaving primary and/or secondary amino groups. Polyamines are essentiallyalkylene polyamines having 1 to 40 carbon atoms. They may carrysubstituents which have no hydrogen atoms that are reactive withisocyanate groups. Examples are polyamines of linear or branchedaliphatic, cycloaliphatic or aromatic structure with at least two aminogroups. Diamines are hydrazine, ethylenediamine, propylenediamine,1,4-butylenediamine, piperazine, 1,4-cyclohexyldimethylamine,1,6-hexamethylenediamine, trimethylhexamethylenediamine,menthanediamine, isophoronediamine, 4,4'-diaminodicyclohexylmethane andaminoethylethanolamine. Preferred diamines are hydrazine, alkyl- orcycloalkyldiamines such as propylenediamines, aminoethylethanolamine and1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane. It is also possible touse, as component (a7), polyamines containing more than two amino groupsin the molecule. In such cases, however, it should be ensured--by usingmonoamines as well, for example--that no crosslinked polyurethane resinsare obtained. Such polyamines which can be used are diethylenetriamine,triethylenetetramine, dipropylenetriamine and dibutylenetriamine.Ethylhexylamine is cited as an example of a monoamine.

As ethylenically unsaturated monomers in the context of the bindercomponent a) it is preferred to use mixtures consisting of from 40 to100, preferably from 60 to 90% by weight of component (i), from 0 to 30,preferably from 0 to 25% by weight of component (ii), from 0 to 10,preferably from 0 to 5% by weight, with very particular preference 0% byweight of component (iii) and from 0 to 50, preferably from 0 to 30% byweight of component (iv) and also from 0 to 5, preferably 0% by weightof component (v), the sum of the weight fractions of (i), (ii), (iii),(iv) and (v) always being 100% by weight.

As component (i) it is possible, for example, to use: cyclohexylacrylate, cyclohexyl methacrylate, alkyl acrylates and alkylmethacrylates having up to 20 carbon atoms in the alkyl radical, suchas, for example, methyl, ethyl, propyl, butyl, hexyl, ethylhexyl,stearyl and lauryl acrylate and methacrylate, or mixtures of thesemonomers.

As component (ii) it is possible for example, to use: hydroxyalkylesters of acrylic acid, methacrylic acid or another α,β-ethylenicallyunsaturated carboxylic acid. These esters can be derived from analkylene glycol, which is esterified with the acid, or can be obtainedby reacting the acid with an alkylene oxide. As component (ii) it ispreferred to use hydroxyalkyl esters of acrylic acid and methacrylicacid in which the hydroxyalkyl group contains up to 6 carbon atoms, ormixtures of these hydroxyalkyl esters. Examples of such hydroxyalkylesters are 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate,2-hydroxypropyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropylmethacrylate, 2-hydroxyethyl methacrylate, 3-hydroxybutyl (meth)acrylateor 4-hydroxybutyl (meth)acrylate. Corresponding esters of otherunsaturated acids, such as ethacrylic acid, crotonic acid and similaracids having up to about 6 carbon atoms per molecule, for example, canalso be used.

As component (iii) it is preferred to use acrylic acid and/ormethacrylic acid. However, it is also possible to use otherethylenically unsaturated acids having up to 6 carbon atoms in themolecule. Examples of such acids are ethacrylic acid, crotonic acid,maleic acid, fumaric acid and itaconic acid.

As component (iv) it is possible, for example, to use: vinylaromatichydrocarbons, such as styrene, α-alkylstyrenes and vinyltoluene,acrylamide and methacrylamide and acrylonitrile and methacrylonitrile,or mixtures of these monomers.

As components (v) it is possible to use compounds which contain at leasttwo free-radically polymerizable double bonds in the molecule. Examplesare divinylbenzene, p-methyldivinylbenzene, o-nonyldivinylbenzene,ethanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate,1,6-hexanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate,pentaerythritol di(meth)acrylate, allyl methacrylate, diallyl phthalate,butanediol divinyl ether, divinylethyleneurea, divinylpropyleneurea,diallyl maleate, etc.

Examples of water-insoluble initiators which can be used arewater-insoluble azo compounds and water-insoluble peroxy compounds.Examples of water-insoluble azo compounds are2,2'-azobis(isobutyronitrile), 2,2'-azobis(isovaleronitrile),1,1'-azobis(cyclohexanecarbonitrile) and2,2'-azobis(2,4-dimethylvaleronitrile). Examples of water-insolubleperoxy compounds are t-amyl peroxyethylhexanoate, t-butylperoxyethylhexanoate, dilauryl peroxide, dibenzoyl peroxide and1,1-dimethyl3-hydroxybut-1-yl peroxyethylhexanoate.

It is of course also possible to add polymerization regulators.

The polymerization of the ethylenically unsaturated monomer or of themixture of ethylenically unsaturated monomers of the binder component a)can be carried out by slowly adding the ethylenically unsaturatedmonomer or the mixture of ethylenically unsaturated monomers to theaqueous polyurethane resin dispersion. In this case it is possibleeither to add the entire amount of the monomers all at once or else tointroduce only a portion initially and to meter in the remaindersubsequently in the course of the reaction. However, the monomers to bepolymerized can also be brought into the form of a preemulsion with theaid of a portion of the polyurethane resin dispersion and water, thispreemulsion then being added slowly to the initial charge. The feed timeof the monomers to be polymerized is generally 2-8, preferably about3-4, hours. The water-insoluble initiators can be added to the initialcharge or added dropwise together with the monomers. They may also beadded fractionally to the initial charge, which contains a portion ofthe monomers. The remainder of initiator is then metered in togetherwith the remaining monomers. The reaction temperature is a function ofthe decomposition rate of the initiator or initiator mixture and can belowered if desired by means of suitable organic redox systems. Thepolymerization of the ethylenically unsaturated monomer or mixture ofethylenically unsaturated monomers takes place generally at atemperature of from 30 to 100° C., in particular at a temperature offrom 60 to 95° C. If operating at superatmospheric pressure, thereaction temperatures may rise above 100° C.

The ethylenically unsaturated monomer or mixture of ethylenicallyunsaturated monomers is to be selected such that the binder component a)obtained in the manner described above has a hydroxyl number of 0-100,preferably 0-80 and an acid number of 10-40, preferably 15-30.

Next, the binder component b) is described in detail below. To prepareit, first of all, a polyurethane resin (B) is prepared in a first stepin accordance with well-known methods of polyurethane chemistry.

The polyurethane resin (B) is prepared from the following components:(b1) a polyester- and/or polyethenepolyol having a number-averagemolecular weight of from 400 to 5000, or a mixture of such polyester-and/or polyetherpolyols; (b2) a polyiso-cyanate or a mixture ofpolyisocyanates; (b3), if desired, a compound which contains not only apolymerizable double bond but also at least one NCO-reactive group aswell, or a mixture of such compounds; (b4), if desired, a compound whichhas in the molecule at least one isocyanate-reactive group and at leastone group which is capable of forming anions, or a mixture of suchcompounds; (b5), if desired, a compound which has in the molecule atleast one NCO-reactive group and at least one poly(oxyalkylene) group,or a mixture of such compounds; and, if desired, (b6) a hydroxyl- and/oramino-containing organic compound having a molecular weight of from 60to 600, or a mixture of such compounds.

The polyurethane resin (B) is intended to have a number-averagemolecular weight of from 200 to 30,000, preferably from 1000 to 5000 andto contain on average from 0.05 to 1.1, preferably from 0.2 to 0.9, withparticular preference from 0.3 to 0.7, polymerizable double bonds permolecule. It is preferred for the polyurethane resin (B) to have an acidnumber of from 0 to 2.0. The molecular weight can--as is known to theskilled worker--be controlled in particular by the proportion and thefunctionality of the starting compounds (b1) to (b6) that are used.

The polyurethane resin (B) can be prepared either in bulk or else inorganic solvents.

A polyurethane resin (B) can be prepared by reacting all startingcompounds simultaneously. In many cases, however, it is judicious toprepare the polyurethane resin (B) in stages. Thus it is possible, forexample, to use components (b1) and (b2) to prepare a prepolymer whichcontains isocyanate groups and is then reacted further with component(b3). It is also possible to use components (b1), (b2), (b3) and, ifdesired, (b4) and (b5) to prepare a prepolymer which contains isocyanategroups and can then be reacted with component (f) to give polyurethanesof higher molecular mass. In cases where a compound used as component(b3) contains only one isocyanate-reactive group, it is possible in afirst stage to use (b2) and (b3) to prepare a precursor which containsisocyanate groups and can then be reacted further with the othercomponents.

The reaction of the components (b1) to (b6) is judiciously carried outin the presence of catalysts, such as dibutyltin dilaurate, dibutyltinmaleate and tertiary amines, for example.

The amounts of component (b1), (b2), (b3), (b4), (b5) and (b6) to beused are a function of the target number-average molecular weight and ofthe target acid number. The polymerizable double bonds can be introducedinto the polyurethane molecules by using components (b1) havingpolymerizable double bonds and/or by means of the component (b3). It ispreferred to introduce the polymerizable double bonds by way of thecomponent (b3).

As component (b1) it is possible to use the compounds described forcomponent (a1) of the polyurethane resin of the binder component a). Allremarks made in relation to component (a1) apply accordingly. The sameapplies to component (b2) in relation to component (a2), to component(b3) in relation to component (a6), to component (b4) in relation tocomponent (a3), to component (b5) in relation to component (a4), and tocomponent (b6) in relation to component (a7).

The ethylenically unsaturated monomers (A) which can be employed in thebinder component b) also correspond in qualitative terms to those asdescribed for binder component a). In detail, however, differentproportions are preferred. In the case of the binder component b) it ispreferred to use mixtures in which component (i) is used in an amount offrom 40 to 100, preferably from 60 to 90% by weight, component (ii) inan amount of from 0 to 20, preferably from 3 to 12% by weight, componentiii) in an amount of from 0 to 30, preferably from 5 to 15% by weight,component (iv) in an amount of from 0 to 30, preferably from 0 to 10% byweight, and component (v) in an amount of from 0 to 5, preferably 0% byweight, the sum of the weight fractions of (i) to (v) always being 100%by weight.

As far as the component (v) is concerned, preference is given inconnection with the binder component b) to the use of difunctionalunsaturated monomers such as butanediol diacrylate or hexanedioldiacrylate. When using glycidyl methacrylate and methacrylic acid, thecorresponding glycerol dimethacrylate is formed automatically during thepolymerization. The nature and amount of polyunsaturated monomers mustbe carefully matched with the reaction conditions (catalysts, reactiontemperature, solvents) in order that no gelling is obtained. The purposeof the amount of polyunsaturated monomers added is to raise the averagemolar mass without instances of gelling. However, it is preferred not toadd any polyunsaturated monomer.

A binder component b) is prepared by preparing a solution of thepolyurethane resin (B) in an organic solvent or an organic solventmixture, polymerizing ethylenically unsaturated monomers or a mixture ofethylenically unsaturated monomers in this solution in a free-radicalpolymerization, and converting the resulting reaction product into anaqueous dispersion. It is preferred to use water-miscible organicsolvents. Examples of solvents which can be used are butyl glycol,2-methoxypropanol, n-butanol, methoxybutanol, n-propanol, ethyleneglycol monomethyl ether, ethylene glycol monoethyl ether, ethyleneglycol monobutyl ether, diethylene glycol monomethyl ether, diethyleneglycol monoethyl ether, diethylene glycol diethyl ether, diethyleneglycol monobutyl ether and 3-methyl3-methoxybutanol or mixtures of thesesolvents. Preference is given to ketones, such as, for example, acetone,methyl ethyl ketone, diethyl ketone and methyl isobutyl ketone.

The free-radical polymerization is conducted at temperatures from 80 to160° C., preferably from 100 to 160° C., in the abovementioned organicsolvents or solvent mixtures. Examples of polymerization initiatorswhich can be used are initiators which form free radicals, such as, forexample, benzoyl peroxide, azobisisobutyronitrile and butylperoxyethylhexanoate. In the course of the polymerization, there arealso grafting reactions between the polyurethane resin (B) and thecomponent (A).

The polymers prepared from (A) and (B) and used in accordance with theinvention must contain groups which are capable of forming anions andwhich, prior to or during the conversion of the polymers from theorganic solvent or solvent mixture into water, are neutralized and allowthe formation of stable aqueous dispersions. The polymers in questionmay include, in addition to the groups capable of forming anions,nonionic stabilizing groups such as, for example, poly(oxyalkylene)groups, especially poly(oxyethylene) and/or poly(oxypropylene) and/orpoly(oxyethylene)-(oxypropylene) groups.

The amount of the groups employed which are capable of forming anionsshould be sufficiently high fox the polymers of the binder component b)to have an acid number of from 5 to 200, preferably from 10 to 40, withparticular preference from 15 to 30. The groups capable of forminganions can be introduced into the polymers in question, for example, byway of the components (b4) and (iii). The groups capable of forminganions can be present exclusively in component (A) or exclusively incomponent (B) or both in component (A) and in component (B). It ispreferred for from 50 to 100, preferably from 70 to 100, with particularpreference 100% of the groups capable of forming anions to be present incomponent (A).

The introduction of poly(oxyalkylene) groups into the polymers of thebinder component b) can take place by way of component (b5) or by way ofethylenically unsaturated monomers which contain at least onepoly(oxyalkylene) group (e.g., poly(oxyethylene) (meth)acrylate). Thepolymers should not contain excessive amounts of poly(oxyalkylene)groups, since otherwise the moisture resistance of the coating films maybe lowered. The poly(oxyalkylene) group content can be from 1 to 10% byweight, preferably from 1 to 5% by weight (based on the weight of thepolymer prepared from (A) and (B)).

The polymers prepared from (A) and (B) which are used should preferablynot contain any nonionic stabilizing groups.

The polymers prepared from (A) and (B) should preferably have a hydroxylnumber of from 0 to 100, with particular preference from 20 to 80. Thenumber-average molecular weight of the polymers should be preferablyfrom 2000 to 20,000, with particular preference from 5000 to 12,000.

Particularly preferred polymers are those polymers prepared from (A) and(B) which have a polydispersity index Q of from 5 to 90, preferably from10 to 30. The polydispersity index is the quotient M_(w) : M_(n), M_(w)being the weight-average and M_(n) the number-average molecular weight.

The polydispersity index can be influenced, for example, by careful useof regulators and by the nature of the solvents employed. In addition, Qis influenced by the polymerizable double bond content of component (B).Q becomes greater the smaller the amount of regulator employed and thesmaller the amount of solvents employed which are able to function asregulators. The lower the polymerizable double bond content of component(B), the greater Q becomes. Q can be determined by means of gelpermeation chromatography using a polystyrene standard.

Once polymerization of component (A) has ended, the polymer obtained issubjected to at least partial neutralization and is dispersed in water.

Neutralization can be carried out using both organic bases and inorganicbases, such as ammonia and hydrazine. It is preferred to use primary,secondary and tertiary amines, such as, for example, ethylamine,propylamine, dimethylamine, dibutylamine, cyclohexyl-amine, benzylamine,morpholine, piperidine and triethanolamine. It is particularly preferredto use tertiary amines as neutralizing agents, especiallydimethylethanolamine, triethylamine, tripropylamine and tributylamine orelse aminomethylpropanol.

Some or, preferably, all of the organic solvents can be removed bydistillation from the resulting aqueous binder dispersions of the bindercomponent b). The binder dispersions b) contain polymer particles whoseaverage particle size lies between 10 and 500 nm, preferably between 60and 150 nm (measurement method: laser light scattering, measurementapparatus: Malvern Autosizer 2 C).

From a binder dispersion of the invention comprising the bindercomponents a) and b) it is possible in accordance with well-knownmethods to prepare aqueous coating materials, especially aqueoussolid-color basecoats and aqueous metallic basecoats. The basecoats canalso be used for refinish purposes and can be overcoated with bothaqueous and conventional clearcoats, powder coating materials and powderslurry coating materials.

In addition to the binder components a) and b), coating materials of theinvention may also include further compatible water-dilutable syntheticresins, such as, for example, amino resins, polyurethane resins,polyacrylate resins, polyesters and polyethers.

Coating materials in accordance with the invention contain preferablyfrom 5 to 90, with particular preference from 40 to 70, % by weight ofbinder component a) plus binder component b), the percentages by weightbeing based on the overall solids content of the basecoats.

As pigments it is possible for basecoats of the invention to containcoloring pigments on an inorganic basis, such as titanium dioxide, ironoxide, carbon black, etc., for example, coloring pigments on aninorganic basis, and customary metal pigments (e.g., commerciallycustomary aluminum bronzes, stainless steel bronzes) and nonmetalliceffect pigments (e.g., pearl luster and interference pigments). Thelevel of pigmentation is within customary ranges. A particular advantageof the binder b) used in accordance with the invention is that it canalso be used as a grinding resin and gives very stable pigment pastes.

Furthermore, coating materials of the invention can have added to themcrosslinked polymeric microparticles, as disclosed, for example, inEP-A-0038127, and/or customary inorganic or organic additives. Forexample, effective thickeners include inorganic sheet silicates,water-soluble cellulose ethers, such as hydroxyethylcellulose,methylcellulose or carboxymethylcellulose and also synthetic polymershaving ionic and/or associative groups, such as polyvinyl alcohol,poly(meth)acrylamide, poly(meth)acrylic acid, polyvinylpyrrolidone,styrene-maleic anhydride or ethylene maleic anhydride copolymers andtheir derivatives or else hydrophobically modified ethoxylated urethanesor polyacrylates and also carboxyl-containing polyacrylate copolymershaving an acid number of from 60 to 780, preferably from 200 to 500.

Basecoats of the invention generally have a solids content of from about15 to 50% by weight. The solids content varies with the intended use ofthe coating compositions. For metallic coating materials, for example,it is preferably from 17 to 25% by weight. For solid-color coatingmaterials it is higher, for example from 30 to 45% by weight. Thecoating materials of the invention may additionally include customaryorganic solvents. The proportion thereof is kept as low as possible. Forexample, it is below 15% by weight.

Coating materials of the invention are generally adjusted to a pH ofbetween 6.5 and 9.0. The pH can be adjusted using customary amines, suchas ammonia, triethylamine, dimethylaminoethanol, N-methylmorpholine andaminomethylpropanol, for example.

Basecoats of the invention can be overcoated with aqueous, conventionalcoating materials, transparent powder coating materials or powder slurrycoating materials.

Using basecoats of the invention, high-quality coating systems can alsobe produced without overcoating with a transparent topcoat. The one-coatsystems obtained in this way are notable in particular for high gloss,good mechanicotechnological properties, and a high level of resistanceto condensation.

Aqueous coating materials of the invention can be applied to any desiredsubstrates, such as metal, wood, plastic or paper, for example.Application can be made directly or, as is usual in the automobileindustry, following the application of an electrodeposition primer and asurfacer. The coating materials of the invention can be applied byspraying, knifecoating, dipping or rolling, preferably by electrostaticand pneumatic spraying.

In the examples which follow the invention is elucidated further. Allparts and percentages are by weight unless expressly stated otherwise.

EXAMPLE 1 Aqueous Dispersion of the Binder Component a)

In a reaction vessel with stirrer and reflux condenser, 173.3 g of apolyesterdiol having a number-average molecular weight of 1480, based ona dimer fatty acid (Pripo1^(R) 1013), isophthalic acid and hexanediolare dissolved, following the addition of 20.2 g of dimethylolpropionicacid and 7.2 g of trimethylolpropane monoallyl ether, in 43.3 g ofN-methylpyrrolidone and 78.5 g of methyl ethyl ketone. Following theaddition of 88.1 g of isophorone diisocyanate, the mixture is heated to80° C. and held at this temperature until the NCO content is 1.8%. Afterthe mixture has cooled to 50° C., 14.4 g of triethylamine, 567.6 g ofdeionized water and 7.3 g of aminoethylethanolamine are added withstirring in rapid succession. The temperature is then raised to 60° C.and the methyl ethyl ketone is distilled off under reduced pressure. Thepolyurethane dispersion obtained in this way has a solids content of 34%and a pH of 8.2.

437.5 g of this polyurethane dispersion are diluted with 275.7 g ofdeionized water. After heating to 85° C., a preemulsion of 50.1 g ofmethyl methacrylate, 50.1 g of styrene, 37.5 g of n-butyl acrylate, 37.5g of hydroxyethyl methacrylate and 77.2 g of the polyurethane dispersionprepared above is added with stirring over the course of 2.5 hours. Withthe commencement of the addition of this mixture, a solution of 2.6 g oft-butyl perethylhexanoate in 32.0 g of methoxypropanol is added over thecourse of three hours. Subsequently, the mixture is held at 85° C. untilthe monomers have been fully consumed by the reaction. If appropriate,further initiator is added. The dispersion obtained in this way, with avery low coagulum fraction, exhibits excellent stability at a solidscontent of 35% and a pH of 7.2.

EXAMPLE 2 Aqueous Dispersion of the Binder Component b)

In a reaction vessel with stirrer, reflux condenser and two feedvessels, 9.3 g of neopentyl glycol, 3.0 g of trimethylolpropanemonoallyl ether, 0.1 g of dibutyltin dilaurate, 110.2 g of methylisobutyl ketone and 63.5 g of isophorone diisocyanate are added to amixture of 77.6 g of a polyesterdiol having a number-average molecularweight of 630 and based on an adipic acid, hexanediol and neopentylglycol. The reaction mixture is subsequently heated to 105° C. At an NCOcontent of 1.8%, 15.1 g of trimethylolpropane are added to the reactionmixture and reaction is continued until isocyanate groups can no longerbe detected.

At a temperature of 105° C., subsequently, a mixture of 69.6 g ofn-butyl acrylate, 69.6 g of methyl methacrylate, 16.6 g of hydroxypropylmethacrylate and 13.0 g of acrylic acid is added to the reaction mixtureover the course of three hours. At the same time, 5.1 g of t-butylperoxyethylhexanoate dissolved in 42.8 g of methyl isobutyl ketone aremetered in over the course of 3.5 hours.

After a further 2.5 hours at 105° C., the reaction mixture is cooled to90° C. Then 10.6 g of dimethylethanolamine and 483.2 g of deionizedwater are added.

Following removal of the methyl isobutyl ketone under reduced pressure,a stable 42% aqueous dispersion having a pH of 7.9 is obtained.

EXAMPLE 3 Solid-color Aqueous Basecoat

200 g of binder component b) as per Example 2 are predispersed in adissolver at 21 m/s for 15 minutes together with 4 g of Surfynolhu R 104(2,4,7,9-tetramethyl-5-decyne-4,7-diol), 50 g of deionized water, 40 gof butyl diglycol, 3 g of a commercially customary defoamer and 250 g ofa white pigment (titanium dioxide), and this initial dispersion is thenground in a bead mill for 30 minutes at a maximum temperature of 50° C.

To 547 g of the above-described millbase there are added 250 g of thebinder component a), 53 g of a commercially customary melamine resin, 30g of butyl glycol, 20 g of Shellsol^(R) T (hydrocarbon mixture,aliphatic, in the range C11-C13 (isoparaffins)), 20 g ofN-methylpyrrolidone, 10 g of 2-ethylhexanol and 70 g of deionized water,with stirring. The pH of the basecoat is subsequently adjusted to 8.3using dimethylethanolamine.

EXAMPLE 4 Comparison of a Coating System Obtained by Means of theCoating Material According to Example 3 with Coating Systems in EachCase on Only one of the Binder Components a) or b)

A coating material according to Example 3 was applied by electrostaticspraying to a substrate of zinc-phosphatized sheet body metal which hadbeen coated with a commercially customary electrodeposition coatingmaterial and a commercially customary surfacer, application giving adry-film thickness of from about 12 to 30 pm. After a short time forevaporation, the system was overcoated with a commercially customaryclearcoat and baked at 130° C. for 30 minutes. A well-leveled finish wasobtained.

Corresponding coating materials were prepared on the basis of the bindercomponents a) and b). In this respect, reference is made individually tothe literature references DE 4010176 and DE P 4339870.7. Coating systemswere prepared in the manner described above using each of the coatingmaterials obtained in this way.

A comparative investigation of the mechanical properties of the coatingsystems obtained in this way showed that a coating system prepared witha coating material of the invention has a considerably higher stone-chipresistance (measureable, for example, by bombardment with 1000 g ofangular chilled cast shot, 4-5 mm diameter, in an Erichsen stone-chipdevice 508 in accordance with VDA [German Automakers Association]) thanthe comparative coating systems based on coating materials with in eachcase only one of the binder components a) or b).

A metal sheet coated as above was overcoated further with a coatingmaterial of the invention (in accordance with Example 3) and, after ashort evaporation period, with a commercially customary clearcoat andwas baked at 130° C. for 30 minutes (high bake refinish) or, after ashort flash-off period, was overcoated with a commercially customarytwo-component refinish clearcoat and dried at 80° C. for 40 minutes (lowbake refinish). A comparative investigation of the mechanical propertiesof the refinish systems obtained in this way with comparison coatingsystems based on the binders a) or b) alone likewise showed a markedlybetter adhesion of the coating system produced with the coating materialof the invention.

What is claimed is:
 1. An aqueous coating material which comprises 5-95%by weight (based on the total weight of the overall binder content) of afirst binder component a) and 5-95% by weight (based on the total weightof the overall binder content) of a second binder component b),the firstbinder component a) being obtained by subjecting a first monomercomponent selected from the group consisting of ethylenicallyunsaturated monomers and mixtures of ethylenically unsaturated monomersto free-radical polymerization in an aqueous dispersion of a firstpolyurethane resin which has a number-average molecular weight of from1000 to 30,000 and which contains on average from 0.05 to 1.1polymerizable double bonds per molecule, in the presence of one or morewater-insoluble initiators, the weight ratio between the firstpolyurethane resin and the first monomer component being in a rangebetween 1:10 and 10:1, and the second binder component b) being obtainedby subjecting(A) a second monomer component selected from the groupconsisting of ethylenically unsaturated monomers and mixtures ofethylenically unsaturated monomers to polymerization in one or moreorganic solvents in the presence of (B) a second polyurethane resin,having a number-average molecular weight of from 200 to 30,000 andcontaining on average from 0.05 to 1.1 polymerizable double bonds permolecule, andconverting the resulting polymer into an aqueousdispersion.
 2. The aqueous coating material of claim 1, wherein thefirst polyurethane resin of the first binder component a) comprises agroup containing polymerizable double bonds selected from the groupconsisting of acrylate, methacrylate and allyl ether functional groups.3. The aqueous coating material of claim 1, wherein the firstpolyurethane resin of the first binder component a) is anionic and hasan acid number in a range from 20 to
 60. 4. The aqueous coating materialof claim 1, wherein the first monomer component of first bindercomponent a) comprises:(i) from 40 to 100% by weight of an estercomponent selected from the group consisting of aliphatic esters ofacrylic acid, aliphatic esters of methacrylic acid, cycloaliphaticesters of acrylic acid, cycloaliphatic esters of methacrylic acid, andmixtures thereof, said ester component containing neither hydroxyl norcarboxyl groups, (ii) from 0 to 30% by weight of an ethylenicallyunsaturated monomer which carries at least one hydroxyl group in themolecule, or a mixture of such monomers, (iii) from 0 to 10% by weightof an ethylenically unsaturated monomer which carries at least onecarboxyl group in the molecule, or a mixture of such monomers, (iv) from0 to 50% by weight of an ethylenically unsaturated monomer other than(i), (ii) and (iii), or a mixture of such monomers, and (v) from 0 to 5%by weight of an ethylenically polyunsaturated monomer or a mixture ofsuch monomers, the sum of the weight fractions of (i), (ii), (iii), (iv)and (v) always being 100% by weight.
 5. The aqueous coating material ofclaim 1, wherein the polymer of the second binder component b), which isprepared from (A) and (B), has an acid number of from 5 to 200, ahydroxyl number of from 0 to 100, and a number-average molecular weightof from 2000 to 20,000.
 6. The aqueous coating material of claim 1,wherein the polymer of the second binder component b) which is preparedfrom (A) and (B) has a polydispersity index Q=M_(w) :M_(n) of from 5 to90.
 7. The aqueous coating material of claim 1, wherein the polymer ofthe second binder component b) is obtained from components (A) and (B)in a weight ratio of from 1:10 to 10:1.
 8. The aqueous coating materialof claim 1, wherein, as component (B) of the binder component b), apolyurethane resin is used containing polymerizable double bonds whichhave been introduced by incorporating compounds containing at least twoNCO-reactive groups into the molecules of the polyurethane resin.
 9. Theaqueous coating material of claim 1, wherein, as component (B) of thebinder component b), a polyurethane resin is used which comprises allylether groups as polymerizable double bonds.
 10. The aqueous coatingmaterial of claim 1, wherein as component (B) of the binder component b)a polyurethane resin is used which comprises trimethylolpropanemonoallyl ether.
 11. The aqueous coating material of claim 1, whereinthe polymer of the binder component b) is obtained using as component(B) a polyurethane resin which has an acid number of from 0 to 2.0. 12.The aqueous coating material of claim 1, wherein the polymer of thebinder component b) is obtained using as component (B) a polyurethaneresin which has a number-average molecular weight of from 1000 to 5000and contains on average from 0.2 to 0.9 polymerizable double bonds permolecule.
 13. The aqueous coating material of claim 1, which furthercomprises one or more pigments.
 14. A method of coating a substratesurface, comprising the steps of:(1) applying a basecoat according toclaim 1 to the substrate surface, (2) forming a basecoat film from thebasecoat applied in step (1), (3) applying a transparent topcoat to theresulting basecoat film, and subsequently (4) baking the basecoat filmtogether with the topcoat film.
 15. The method of claim 14 wherein thesubstrate comprises a motor vehicle body surface.
 16. The aqueouscoating material of claim 5, wherein the polymer of the second bindercomponent b) has an acid number of from 10 to 40, a hydroxyl number offrom 20 to 80, and a number-average molecular weight of from 5,000 to12,000.
 17. The aqueous coating material of claim 6, wherein the polymerof the second binder component b) has a polydispersity index M_(w):M_(n) of from 10 to
 30. 18. The aqueous coating material of claim 7,wherein the polymer of the second binder component b) is obtained frompolymerizing components (A) and (B) in a weight ratio in a range from1:2 to 2:1.